1. Field of the Invention
The present invention relates to a Multiple Input Multiple Output (MIMO) wireless communication system. More particularly, the present invention relates to a device and a method for eliminating inter-cell interference in a MIMO wireless communication system.
2. Description of the Related Art
As the demand for high quality data transfer with high speed has increased, Multiple Input Multiple Output (MIMO) technology, which uses a plurality of transmitting and receiving antennas, has received increasing attention as one of technologies for satisfying the demand. According to the MIMO technology, communication is performed using a plurality of channels due to the plurality of antennas so that a channel capacity may be improved in comparison with a single antenna system.
The MIMO technology is divided into Closed Loop (CL) MIMO and Open Loop (OL) MIMO according to whether channel information of user equipment is used when a transmission beam pattern is formed. In the case of the CL MIMO, the user equipment feedbacks the channel information to a base station, and the base station forms an appropriate transmission beam based on the channel information to transmit a signal. In the case of the OL MIMO, there is no feedback of the channel information, and thus the base station uses one fixed beam pattern or a plurality of fixed beam patterns regardless of channel state.
As one method of embodying the CL MIMO, a method of feeding back a codebook vector index or a codebook matrix index is widely used. According to the above-mentioned method, a codebook including codebook vectors is predefined. Herein, each codebook vector has a direction which maximizes a gain in space. According to the method, the user equipment provides the codebook vector index or codebook matrix index which maximizes Signal to Noise Ratio (SNR) to the base station. In other words, the user equipment measures a channel using a training signal received from the base station and selects a beamforming vector satisfying a desired SNR. Thereafter, the user equipment feedbacks an index of the selected beamforming vector, i.e., a beam index, to the base station, and the base station transmits a signal using the beam index fed back to the user equipment. Accordingly, a high SNR may be achieved and throughput may be maximized.
However, since all base stations use the same codebook, in the case where neighboring base stations transfer data using the same frequency band, the user equipment positioned at a boundary of a cell may be interfered due to the beam index used by a neighboring cell user equipment positioned at the boundary of the neighboring cell. For instance, in the case where the direction of the beamforming vector used for a base station A to transfer data to a user equipment A positioned at the cell boundary using a first frequency band and that of the beamforming vector used for a base station B to transfer data to a user equipment B positioned at the cell boundary using the first frequency band are overlapped, signals received by the user equipments A and B mutually interfere with each other.
To address such a problem, standards organizations and scholars have been conducting research on technology for controlling Inter-Cell Interference (ICI) through cooperation between base stations. The technology of cooperation between base stations is referred to using different names according to the standards organizations. The technology of cooperation between base stations is referred to as Coordinated Multiple Point Transmission/Reception (CoMP) technology in the 3rd Generation Partnership Project (3GPP) Long Term Evolution-Advanced (LTE-A) standard, and it is referred to as multi-cell MIMO technology in the Institute of Electrical and Electronics Engineers (IEEE) 802.16m standard.
The CoMP technology is roughly divided into Coordinated Beamforming (CB) technology and Joint Processing/Transmission (JP) technology. According to the CB technology, like a conventional single-cell MIMO technology, a traffic data for a single user equipment is transmitted from a single base station. Accordingly, the CB technology does not need packet sharing between cells. However, reception SNR increasing technology such as transmission combining cannot be used. On the contrary, according to the JP technology, the traffic data for the single user equipment is transmitted from two or more base stations. In this case, the SNR may be increased using the reception SNR increasing technology, but the packet sharing between cells is needed.
As a representative example of the CB technology, there is Precoding Matrix Index (PMI) coordination technology. According to the PMI coordination technology, precoders applied to neighboring cells are coordinated based on information fed back to respective serving base stations from the user equipments so that the inter-cell interference is reduced. According to the PMI coordination, the user equipment searches for the most interfering PMI (hereinafter, referred to as worst PMI) or the least interfering PMI (hereinafter, referred to as best PMI) and feedbacks them to the serving base station. At this time, the serving base station sends a PMI coordination request to the neighboring base station to inform the neighboring base station of the worst PMI or the best PMI fed back by the user equipment so that the use of the worst PMI may be restricted or the use of the best PMI may be recommended. Accordingly, the neighboring base station does not use the worst PMI or use the best PMI. Therefore, the inter-cell interference may be reduced. However, the PMI coordination technology has several problems.
Firstly, ideal performance may be obtained under the ideal condition that signaling is performed between the user equipment at the cell boundary and the serving base station or between base stations using respective infinite bandwidth. However, in an embodiment which uses limited bandwidth, all of interfering subbands and PMIs cannot be eliminated, and thus residual interference exists. Accordingly, it is difficult to obtain the ideal performance. Therefore, more efficient signaling is needed.
Secondly, as the user equipment at the cell boundary sends the PMI coordination request to all neighboring interfering cells, the user equipment at another cell boundary sends the PMI coordination request. Like this, if each user equipment at the cell boundary sends many PMI coordination requests, the base station also receives many PMI coordination requests from the user equipments of other cells. Therefore, all base stations become short of band/PMI for serving their own user equipments. That is, a conflict exists between an aspect of the serving cell and that of the other cell.
Thirdly, on the contrary, in the case where the interfering cell accepts too few PMI coordination requests from the interfered cell, the interfering cell still uses a band/PMI which interferes with the neighboring cells, and thus a PMI coordination gain is small.
Therefore, there is a needed to efficiently reduce the inter-cell interference in order to reduce a conflict between cells.